Settled dust sampling

There are multiple approaches of how to collect settled dust, such as active collection of dust settled from elevated surfaces with a vacuum cleaner (eg. Jayaprakash et al. 2017), and sampling airborne dust passively onto a sampler utilizing electrostatic forces of a sampling surface (electrostatic dust fall

collectors or EDCs) (Noss et al. 2008). In the following, I will introduce the approaches relevant to my study, i.e. Petri dish sampling and REPS-sampling for the collection of settling airborne particles.

2.2.1 Petri dish sampling

Petri dish sampling has been suggested to be a practical and reliable sampling approach in multiple studies, and it is a commonly used sampling approach for settled dust. For example, Adams et al. (2015) noticed in their study, that Petri dish approach for settled dust sampling coupled with DNA-based microbial determinations yielded high correlations between duplicate samples, and therefore has potential to be a useful sampling approach in epidemiological studies. In the same study, it was concluded that Petri dishes are an inexpensive sampling approach, and it also indicated highest quantitative determinations compared to for example EDC’s, which are also widely used samplers in settled dust sampling.

Leppänen et al. (2017) suggested Petri dish approach to reflect well the microbial composition of indoor air, but they recommended other sampling approaches to be assessed in parallel for better quantitative assessments (for example dust reservoir samples). Similar conclusions have been made in other studies too, such as in Karlsson et al. (2002). In their study, these authors noticed significant correlation between Petri dish sampling and personal air sampling in the analysis of cat allergens (rho=0.66; p<0.0001). These results indicate that Petri dish sampling can be a more practical approach than active air sampling, when long term exposure to airborne particles is of interest.

Petri dish sampling has become a popular settled dust sampling approach not only due to the indications of its repeatability and reliability, but also due to some practical features. First of all, Petri dishes are inexpensive to purchase, and their small size and light weight is practical for field work. Also, post-sampling and pre-analysis processing of the settled dust collected in Petri dishes is relatively simple and quick, in that no time-consuming steps or expensive equipment are required. Extraction of particulate matter from EDCs for example involves multiple processing steps including a stomacher or shaker for particle release and centrifugation to concentrate the large amounts of buffer required for an efficient extraction (Adams et al. 2015).

In contrast, settled dust from Petri dishes can be collected quickly onto a cotton swab that then can be further extracted for analysis. Settled dust sampling with Petri dishes itself can be done easily without

trained personnel, which is particularly important in large epidemiological studies that require study participants themselves to carry out indoor environmental sampling campaigns. These practical

advantages alongside the apparent good repeatability and correlation with active air sampling highlights Petri dish sampling of settled dust as a useful, promising indoor microbial sampling approach.

2.2.2 Challenges in settled dust sampling

Even though Petri dish sampling has been used as a sampling method to assess the long term microbial exposure, there are some challenges regarding the Petri dish, and also settled dust sampling approaches in general.

One issue regarding settled dust sampling approaches is the resuspension of settled dust particles. This is discussed in a study by Adams et al. 2015, and they hypothesize, that the resuspension of settled dust depends on the samplers’ material. With extended sampling periods the sampler can become saturated with dust, and this can prevent further sampling to be accurate. Also, settled dust sampling is rarely conducted in a space where absolutely no air flow is present. The air flow in the space where settled dust sampling is conducted is problematic, because it can cause resuspension of settled dust from smooth-surfaced samplers (Frankel et al. 2012). Therefore, settled dust sampling is not as efficient in a space where air flow is present, and this should be considered when appropriate sampling location is chosen in a sampling space.

Another problematic feature for settled dust sampling is the difference in sampling efficiencies between PM2.5 and PM10. In their study, Kilburg-Basnyat et al. 2016 noticed, that EDCs collected PM2.5 particles more effectively than PM2.5-PM10 particles. At the same time it is known, that the coarse particulate matter (PM2.5-PM10) contain approximately 10 times higher concentrations of endotoxin compared to the fine particulate matter (PM2.5) (Heinrich et al. 2003). These factors can result into unrealistic

measurement results of endotoxin, and other biological factors when EDCs, or settled dust sampling approach in general, is used. More research is needed in order to understand the sampling efficiencies of different particles for individual settled dust sampling approaches.

In several previous studies carried out in the research group at THL, it has been noticed that specifically Gram-positive bacterial levels detected from outdoor locations sampled with Petri dishes, followed up by qPCR-method, returned implausibly low results. This problem has not been studied in further detail, but

there are some potential explanations for this issue. One of these explanations could relate to the

sampling approach that would make Gram-positive bacteria susceptible to degradation on the petri dish surface. More likely, however, are issues with the Gram-positive bacterial probe in the quantitative PCR assay used in studies (Kärkkäinen P, Valkonen M, et al. 2010).

2.2.3 REPS-sampling

The REPS is the least studied of all the sampling approaches discussed in this thesis, which motivated us to carry out a targeted study. This sampling approach has been introduced by Therkorn et al. (2017), where they compared the REPS with Button Inhalable sampler, PTFE (Polytetrafluoroethylene) settling filters and Agar settling plates. The REPS is a passive dust sampler, which has electrostatic properties due to the polyrized PVDF (poly(vinylidenefluoride)) film that is utilized in its structure. Therefore, the sampling of airborne particles is not depending solely on particles settling under gravity, but also on the electrostatic forces attracting the airborne particles to attach on the PVDF film utilized in the REPS.

Because of these electrostatic properties of the REPS, the sampling efficiency is supposedly higher than it is for example with Petri dish sampling approach (Thekorn et al. (2017)).

The REPS is designed in a way that the sampling surface area is as large as possible at a relatively small sampler size. This has been achieved with using a spiral shape for the REPS. The REPS has also been designed to be as practical as possible in laboratory analysis, since the shape of the sampler is designed in a way that it fits into a 50mL conical centrifuge tube. This eases the processing before and after sampling campaigns (Therkorn et al. (2017)). The structure of the REPS is presented in picture 1.

In their study, Therkorn et al. (2017) noted some promising results about the REPS. They discovered that the REPS provided greater passive collection of total bacteria and fungi compared to passive PTFE settling filters, and settling agar plates. This was thought to be the result of the electrostatic properties of the REPS, which increases the sampling efficiency compared to the sampling methods relying only on the gravitational settling of airborne particles. When Therkorn et al. (2017) compared the REPS sampling approach with active air sampling using Button Inhalable samplers they discovered, that the REPS had enhanced performance in culturable bacteria collection.

The most important result Therkorn et al. (2017) found from comparison of the REPS and active air sampling was the higher practicality of REPS compared to active air sampling, with the REPS collecting

airborne particles relatively efficiently because of the electrostatic forces created by PVDF film, without the need for air flow, sampling pumps and personnel operating the equipment in the field.

Picture 1. Front and top views of the REPS film holder (Therkorn et al. (2017)). PVDF (polyvinyliudene fluoride) film is spiraled through the film holder openings, as presented in the top view of the picture.